The present invention relates to a semiconductor optical element having a layer which exhibits a function of diffraction grating, and to a process for producing the same.
FIG. 1 shows a conventional semiconductor optical element such as a semiconductor laser element having a layer which exhibits a function of diffraction grating. Namely, FIG. 1 is a section view showing the structure of layers of, for example, a heterostructure laser, or a GaAs/AlGaAs distributed feedback (DFB) type laser disclosed in Nakamura et al, Appl. Phys. Lett. 27, 403 (1975). In FIG. 1, reference numeral 1 denotes a first electrode, 2 denotes a GaAs substrate to one surface of which is connected the first electrode, 3 denotes a first cladding layer which consists of an n-type AlGaAs layer formed on the upper surface of the substrate 2, reference numeral 4 denotes an active layer which consists of a GaAs layer formed on the upper surface of the first cladding layer, 5 denotes an optical guide layer which consists of an AlGaAs layer and which has a diffraction grating 6 formed by periodically engraving grooves, 7 denotes a second cladding layer which consists of a p-type AlGaAs layer formed on the upper surface of the optical guide layer 5 via the diffraction grating 6, reference numeral 8 denotes a cap layer which consists of a p-type GaAs layer formed on the upper surface of the second cladding layer, 9 denotes a second electrode formed on the upper surface of the cap layer, and 10 denotes a laser beam.
The operation will be described below. If an electric current greater than a threshold value is supplied across the first electrode 1 and the second electrode 9, injected carries (electrons and positive holes) are confined in the active layer 4 and recombine to emit light which propagates through an optical waveguide sandwiched by the first and second cladding layers 3, 7. At this moment, only the light having a particular oscillation wavelength .lambda.(=2n.LAMBDA./l) (where n denotes a refractive index of the optical waveguide, and l denotes a degree) defined by a period .LAMBDA. of the diffraction grating is amplified by positive feedback to undergo laser oscillation having an intense wavelength selectivity, owing to the optical guide layer with diffraction grating 6 having a sawtoothed periodic distribution of refractive indexes formed by engraving the grooves.
The conventional semiconductor optical element having a layer which exhibits a function of diffraction grating is produced by a method shown in FIGS. 2(a) to 2(d). First, as shown in FIG. 2(a), on the substrate 2 are epitaxially grown the first cladding layer 3, active layer 4, and optical guide layer 5. Next, as shown in FIG. 2(b), on the optical guide layer 5 is formed a periodically masked diffraction grating pattern 11 by, for example, holographic interference lithography, electron beam lithography, or a like method. Relying upon a chemical etching or dry etching effected through the diffraction grating mask pattern 11, sawtoothed grooves are formed in the optical guide layer 5 as shown in FIG. 2(c) and, then, the diffraction grating mask pattern 11 is removed. Thereafter, the second cladding layer 7 and the cap layer 8 are formed as shown in FIG. 2(d) by the epitaxially growing method of the second time, and the first and second electrodes 1, 9 are formed, thereby to complete a semiconductor optical element.
The layer having diffraction grating was formed by engraving the grooves as described above. Therefore, it was difficult to periodically engrave the grooves maintaining good reproduceability and efficiency. Moreover, the crystal had to be grown for the second time on the grooves having a step of about several thousand angstroms, causing the crystallinity of the upper layer to deteriorate. Further, many crystalline defects were inevitably introduced into the interface between the grooves and the upper layer which is the second cladding layer 7.